Evolutionary Trends Within The Mollusca

A spectacular variety of mollusk morphotypes and life modes evolved during the Phanero-zoic, from the simple body plan of the arche-mollusk. Despite the diversity of early mollusks in the Cambrian, the phylum was not notably conspicuous in the tiered suspension-feeding

Box 13.8 Gradualistic evolution of belemnites

There are relatively few long fossil lineages that can be used to demonstrate either phyletic gradualism or punctuated equilibria. Most of the best case studies (see p. 124) are based on mobile or sessile benthic organisms. The Cretaceous (Campanian) belemnite faunas, particularly the genus Belem-nitella, of North Germany are abundant, well preserved and known in great detail and provide an unequalled opportunity to test these models using a pelagic group of organisms (Christensen 2000). There is not much variation in lithology throughout the succession in Lower Saxony - they are mainly rather boring, monotonous marly limestones. There are thus limited opportunities for facies shifts to influence the morphological record of Belemnitella by the migration of more exotic morphotypes in and out of the basin. Although samples of Belemnitella through the section are superficially similar, several measurements show gradualistic trends when treated quantitatively (Fig. 13.24). Not all changes are unidirectional, some exhibit reversals. It seems most likely that the Campanian belemnites of northern Germany conformed to continuous, gradual phyletic evolution in narrowly fluctuating, slowly changing environments.

Figure 13.24 Gradualistic evolution of Cretaceous belemnites from North Germany. Summary of changes of the length from the apex to the protoconch (LAP), Birkelund index (BI), Schatzky distance (SD), fissure angle (FA) and alveolar angle (AA) of nine samples of Belemnitella. Successive mean values are different at the 5% level (one arrow), 1% level (two arrows) and 0.1% level (three arrows). (Courtesy of the late Walter Kegel Christensen.)

Figure 13.24 Gradualistic evolution of Cretaceous belemnites from North Germany. Summary of changes of the length from the apex to the protoconch (LAP), Birkelund index (BI), Schatzky distance (SD), fissure angle (FA) and alveolar angle (AA) of nine samples of Belemnitella. Successive mean values are different at the 5% level (one arrow), 1% level (two arrows) and 0.1% level (three arrows). (Courtesy of the late Walter Kegel Christensen.)